METHODS OF ISOLATION. 



II 



quite constantly, but merely as followers of something else. When possible, 

 therefore, diseased plants should be examined for the suspected pathogen, in large 

 numbers, in different years, and from widely separated localities. Of course, if fungi 

 are also present they must likewise be examined as to constant occurrence and 

 pathogenic properties. 



Under (/>) all of the standard nutrient media should be tried, and that repeatedly, 

 until the student is entirely familiar with the appearance and behavior of the 

 organism. It is usually best to isolate the organism for experiment from selected 

 portions of the tissue by means of Esmarch roll-cultures or by the use of poured 

 plates (Petri-dish cultures), generally the latter. 



Isolations ma}' also be made by inserting a sterile platinum needle or loop into 

 the diseased tissue, obtaining therefrom a little fluid, and drawing this over the 



Fig. 6.* 



surface of slant agar, gelatin, or potato a number of times. This is an old method 

 introduced by Koch in 1881. If ten or twelve tubes are used, the final streaks will 

 often consist only of scattering colonies, from one or more of which the subcultures 

 may be made. The plate method has the great advantage of showing just how 

 many kinds of bacteria are present in the tissues (provided they will all grow in the 

 medium used and under the conditions of the experiment), and just how numerous 

 they are. In case of viscid organisms, or those forming compact zooglcese in the 



*Fic. 6. Cross-section of root of plant No. 53 (turnip) parasitized by Bacterium campestre, 

 showing an early stage in the formation of a bacterial cavity. The original section was made from 

 material fixed in alcohol, infiltrated with paraffin, stained with carbol-fuohsin, and washed in a mix- 

 ture of alcohol and water. Drawn from a photomicrograph. X 500. 



